Found this:

Wallace Racing - Braking HP Loss at Altitude Calculator

500hp at sea level = 482 at 1200' = 464 at 2400' = 446 at 3600'... linear (18hp/1200')? Really? That doesn't seem right.

I'm pretty sure my numbers are largely due to -me-. I'll get better.

Crazy number of calculators at the site: Wallace Racing - Automotive Calculators

Remember to that there is about a 18 to 20% difference between bhp and whp...whp being the lesser of the two.

Right but I was just investigating the affect of altitude on HP; was it linear or not. It appears to be linear which doesn't feel right but hey...who am I.

10-4, I'm not an engineer nor did I sleep at a Holiday Inn Express!!!

That peak hp calculator is a gross over simplification of reality as it is based only on standard atmosphere with elevation. Also, that calculator does not really apply to your car since you have turbos which will just work harder (up to a point) to make up the pressure loss from increased altitude and temperature (aka density altitude). It may take your turbos a little longer to get peak boost and peak torque and therefore it will feel less responsive in the low and mid range rpms, but generally speaking your peak HP numbers will be much less affected by changes in altitude when compared to us lesser NA folks (and even supercharged vehicles to lesser degree). Superchargers are mechanically driven so they will only increase manifold pressure by a fixed amount relative to the outside air pressure. Your turbos will keep working until they hit the wastegate spring pressure (regardless of the outside air pressure).

As I alluded to, density altitude is the more relevant factor in assessing power loss of an engine as it accounts for changes in air density from both altitude and temperature. Density altitude is not linear but its kinda close. At 1000ft elevation and a temp of 70F, the density altitude is more like 2000ft. Humidity will also rob power as the air mass is replaced with water vapor at the same pressure.

Outstanding explanation and according to my butt dyno is spot on. It's weird how on some days my car's responsiveness is like a dog until it gets wound up and then other days she just move out like being at a lower elevation. I assumed that is was mostly a factor of atmospheric conditions. Humidity and high temps are killer period!

Thanks and repped...

seeing that makes me feel accomplished since i still have a better time than most of the guys at lower almost sea level areas lol :stirthepot:

By the way you should try the da calc on dragtimes

Cool! This has density.

DA Calculator - Density Altitude Calculator - DragTimes.com